The present invention is a continuation-in-part of my copending application Ser. No. 597,285, filed July 18, 1975, now abandoned.
The present invention relates to a hole-drilling, extruding and thread-forming sheet screw for use with a workpiece in the form of a metallic sheet or relatively thin metal plate, and more particularly to improvements in a sheet screw which is so designed as to initially form a small pilot hole in the workpiece by self-drilling, then form a tubular projection or extrusion by forcibly enlarging the drilled hole and deforming the metal around said hole, and finally form female threads internally with the formed extrusion by self-threading.
Generally, the hole-drilling, extruding and thread-forming sheet screws should be firmly fastened into or gripped by the workpiece. For that purpose, the following four requirements should be met, the first being that the axial length or height of a formed tubular projection or extrusion should be as large as possible, so that the formed extrusion can be internally formed with a maximum number of female threads; the second being that the wall thickness of the formed extrusion should not be so reduced as to lose its rigidity; the third being that undesirable cracks or splits in or around the formed extrusion should be prevented from development during axial feeding and rotation of the sheet screw; and the fourth being that the internal diameter of the formed extrusion should not be excessively enlarged by vibration during rotation of the sheet screw. Further, as the fifth requirement, from the viewpoint of a product appearance, the sheet screw should be designed so that it can be efficiently driven into the workpiece without such a great axial force as to cause an undesirable concave deformation in the workpiece.
However, none of the conventional hole-drilling, extruding and thread-forming sheet screws are provided with all of the above-discussed five requirements. For example, a hole-drilling and thread-forming screw as disclosed in Australian Pat. No. 247,297 to Kahn, which is considered to be one of the most relevant prior art patents in the field, does not meet the first thru the third requirements. The screw according to Kahn comprises a substantially bulletnosed pilot end section which includes a front drilling section with cutting edges and a conical extruding section with male threads, and a cylindrical body section with male threads. Because the male threads disposed on the conical extruding section will cut the internal wall of a drilled hole in the workpiece after the drilling operation by the drilling section has been finished, the wall thickness of the formed extrusion is excessively reduced, and undesirable cracks or splits are easily developed.
Another example of the conventional hole-drilling, extruding and thread-forming screw is disclosed in U.S. Pat. No. 3,438,299 to Gutshall, which is also considered to be one of the most relevant prior art patents. The screw according to Gutshall comprises a drilling section including a tip portion and a substantially flat portion, an extruding section with a spiral flute having sharp edges for easy penetration into the workpiece, and a cylindrical thread-forming section with male threads, said extruding section being defined by one-half of a bulletnosed configuration generally triangular in cross section and thus non-symmetrical. When this type of screw is driven into the workpiece at a relatively high rotative speed (usually about 2,000 to 2,500 by an electric screw driver, and about 4,000 to 5,000 by a pneumatic screw driver), it has proven that considerably large vibrations are generated due to the non-symmeticical construction of the extruding section. This results in producing an initially formed hole to be excessively enlarged in diameter, and promotes development of cracks or splits caused by the sharp edges of the spiral flute. In other words, the screw of Gutshall does not meet the afore-mentioned third and fourth requirements. Further, the peculiar and specific construction of the Gutshall screw invites a high manufacturing cost, as will be easily understood.
A further example of the conventional hole-drilling, extruding and thread-forming screw is disclosed in U.S. Pat. No. 3,724,315 to Sygnator, which is also considered to be one of the most relevant prior art patents in the field. The screw according to Sygnator comprises a front drilling section with sharp cutting edges, a conical extruding section which is generally elliptical in cross section, and a cylindrical thread-forming section with male threads, wherein the central axis of said conical extruding section is out of alignment with or in other words not coincident with the axis common to the drilling section and the thread-forming section, and said sharp cutting edges extend into the conical extruding section. Thus, the non-symmetrical (elliptical) construction and the eccentrically disposed central axis of the conical extruding section cause considerable vibrations when the screw is driven into the workpiece at a relatively high rotative speed thereby to cause the hole in the workpiece to be excessively enlarged in diameter. Further, because the sharp cutting edges extend from the front drilling section up into the conical extruding section, not only a relatively large hole is formed by drilling thereby to reduce the axial length or height of the formed extrusion, but also the particular construction permits chips produced by drilling to remain on the conical extruding section. Those chips cannot be removed easily, because the gaps between the internal circular surface of the drilled hole and the external circular surface of the conical extruding section are so small since the latter section is substantially circular in cross section. As a result, the internal wall of the formed extrusion is injured and cut by the produced chips during a high speed rotation of the screw thereby to cause reduction in the wall thickness of the extrusion as well as development of cracks or splits in and/or around the extrusion. Further, it should be noted that a bottom portion of the conical section, which initially comes into contact with the marginal edge of the drilled hole, is considerably large in radius and area, and this naturally causes insufficiency in a desired stress concentration required for effectively enlarging the drilled hole. Thus, deforming the metal around the hole cannot be obtained because of stress dispersion, resulting in that a sufficiently high or long extrusion cannot be formed. Further, the insufficiency in the stress concentration, in other words stress dispersion, invites such a disadvantage that a great axial force is required for feeding the screw axially into the workpiece, which, in turn, causes not only an undesirable concave deformation in the workpiece but also insufficiency and fatigue in operation since an operator must apply a great physical force. Thus, it is comprehensible that the Sygnator's screw is dissatisfactory in all of the aforementioned first thru fifth requirements.
Though not relating to a hole-drilling, extruding and thread-forming sheet screw, U.S. Pat. No. 2,703,419 to Barth is considered to be one of the relevant prior art patents in the field, wherein a combination tool is disclosed which is designed so as to cooperate with a die having an orifice therein for forming a tubular extrusion and then forming internal threads in the formed extrusion. This tool comprises a piercing (drilling) front section, an extruding section with spherical, conical surface, and a thread-forming section with male threads, wherein the extruding section is radially enlarged quite abruptly and thus the configuration line defining the drilling and the extruding sections is not rectilineal. In other words, the drilling section is not transformed into the extruding section quite smoothly, but provides a substantial step therebetween. Thus, when the bottom area of the extruding section comes into the marginal wall of a hole formed by the drilling section, there occurs a considerably great resistance against the axial penetration of the extruding section into the workpiece. Thus, in order to overcome this resistance, a correspondingly great axial force must be applied for the penetration of the extruding section into the workpiece, thereby causing an undesirable concave deformation in the workpiece as well as poor efficiency and an operator's fatigue in operation. This problem can be ignored in tool set forth in the Barth Patent, because it is a combination tool cooperating with the die. However, if the idea of this combination tool is applied to a sheet screw, the problem is quite significant, particularly in the case where workpiece is a relatively thin or is a soft metallic sheet which can be easily deformed concavely. Further, as substantially same with the above-discussed screw of Sygnator, the Barth's tool, if utilized as a sheet screw, has such a disadvantage that the bottom portion of the extruding section, which comes into contact with the marginal wall of a drilled hole, is so large in radius and contacting area that the stress required for forcibly enlarging the drilled hole and then forming a desired extrusion is inevitably dispersed, thereby hindering effective penetration of the extruding section into the workpiece. Namely, if the Barth's tool is applied to the hole-drilling, extruding and thread-forming sheet screw, it does not meet the afore-mentioned fifth requirement.
It is, therefore, a primary object of the present invention to provide an improved hole-drilling, extruding and thread-forming sheet screw designed so as to meet all of the above-discussed five requirements.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of some of the preferred embodiments thereof taken in conjunction with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as defining the limits of the present invention.